The miniaturization of electronic devices has created a demand for small but powerful electrochemical cells. Cells that utilize an alkaline electrolyte are known to provide high energy density per unit volume and are therefore well suited for applications in miniature electronic devices such as hearing aids, watches and calculators. However, alkaline electrolytes, such as aqueous potassium hydroxide and sodium hydroxide solutions, have an affinity for wetting metal surfaces and are known to creep through the sealed metal interface of an electrochemical cell. Leakage in this manner can deplete the electrolyte solution from the cell and can also cause a corrosive deposit on the surface of the cell that detracts from the cell's appearance and marketability. These corrosive salts may also damage the device in which the cell is housed. Typical cell systems where this problem is encountered include silver oxide-zinc cells, nickel-cadmium cells, air depolarized cells, and alkaline manganese dioxide cells.
In the prior art it has been a conventional practice to incorporate insulating gaskets between the cell cup and can so as to provide a seal for the cell. Generally, the gasket must be made of a material inert to the electrolyte contained in the cell and the cell environment. In addition, it must be flexible and resistant to cold flow under the pressure of the seal and maintain these characteristics so as to insure a proper seal during long periods of storage. Materials such as nylon, polypropylene, ethylene-tetrafluoroethylene copolymer and high density polyethylene have been found to be suitable as gasket materials for most applications. Typically, the insulating gasket is in the form of a J-shaped configuration in which the extended wall of the cup is inserted so that upon being radially squeezed, the gasket forms a seal for the cell. The gasket generally extends the entire length of the internal wall of the cell. The volume of the gasket is generally in excess of 20% of the internal volume of the cell and therefore results in a waste of space in the cell for the active components of the cell. To better insure a good seal, a sealant is generally applied to the gasket, including its U-shaped groove, so that upon insertion of the cup into the gasket the edge of the extended wall of the cup will seat in the sealant, and then upon the application of a radial squeeze, the gasket will be compressed against the edge of the extended cup wall.
U.S. Pat. No. 4,302,517 discloses a sealed galvanic cell employing an insulating gasket between the can and the cup of the cell. The cell is composed of a first sealing segment disposed and compressed between the rim of the can and the edge of the cup and a second can support segment extending within the cup and substantially parallel to the wall of the cup and defining a plurality of spaced apart openings which accommodate the cell's electrolyte and/or the cell's reaction product.
In conventional zinc alkaline cells, the zinc component contacts the terminal, which generally has a large surface area. The zinc in the cells, preferably mercury free cells, forms reactive gases, such as hydrogen, which are detrimental to the proper operation of the cells.
It is an object of the present invention to provide a miniature cell structure that employs an insulating member and current collector assembly, such as a rigid plastic member with a low surface area current collector member.
It is another object of the present invention to provide a current collector member with a minimum surface area to prevent or minimize the formation of gases, such as hydrogen, on the current collector member.
It is another object of the present invention to provide an insulator for a miniature cell in which the insulator has a groove to accommodate the peripheral flange of the cup of the cell's housing to provide a sealed miniature cell.
It is another object of the present invention to provide a process for producing a miniature cell with a novel insulator and current collector assembly.
The foregoing and additional objects of the present invention will become more fully apparent from the following description and accompanied drawings.